Typically natural gas transmission pipelines operate at pressures ranging between 700 and 1500 psia. Natural gas pressure reduction points are often referred to as let-down stations. Such stations enable the regional distribution of natural gas (typically at pressures of 150 to 500 psia). In general, let-down stations are not designed for the useful recovery of the pressure energy. Processes which serve to let-down natural gas while producing a fraction of the inlet gas as liquefied natural gas are often referred to as expander cycles or expander plants.
Typically, natural gas is transmitted with residual amounts of water 5–10 lbs-H2O/MMscfd and about 2.0 mole % carbon dioxide or more. In order to operate a cryogenic process (such as an expander plant) producing liquefied natural gas from a pipeline gas, it is necessary to remove both the water and the carbon dioxide to very low levels (<1 and <50 ppm, respectively). The removal of high boiling contaminants (water, carbon dioxide, hydrogen sulfide) is often referred to as pre-purification or pre-treatment. Adsorption systems are often used for the removal of water, carbon dioxide and hydrogen sulfide from pipeline gas streams. The regeneration of adsorption systems requires that a cleaned (contaminant free) stream be passed over the loaded bed in order to remove the high-boiling contaminants. Typically, regeneration gas for these systems is derived from the compression of low-pressure flash gas. This flash gas is generated upon the depressurization of highly subcooled supercritical pressure natural gas. Such an approach results in poor liquefaction efficiency and low liquefied natural gas yield (typically <10% of the feed is liquefied).
Accordingly it is an object of this invention to provide an improved method for producing liquefied natural gas using subambient expansion.